For testing purposes I deleted a Site Collection and tried to restore it in the same Content Database using the Restore-SPSite cmdlet. But then I got the following error message:

Sorry, something went wrong
The operation that you are attempting to perform cannot be completed successfully. No content databases in the web application were available to store your site collection. The existing content databases may have reached the maximum number of site collections, or be set to read-only, or be offline, or may already contain a copy of this site collection. Create another content database for the Web application and then try the operation again.

Navigate to Central Administration > Monitoring > Job definitions and run the “Gradual Site Delete” job for the web application you are trying to restore the Site Collection. Wait a couple of minutes (or at least until the timer job has finished) then try the restore again.